a) Field of the Invention
The present invention relates to a semiconductor device and its manufacturing method, and more particularly to a semiconductor device having insulated gate type field effect transistors (IGFET) fabricated at a high integration.
b) Description of the Related Art
As patterns of large scale integration (LSI) circuits are becoming finer, it is desired to improve a pattern transfer precision. Anisotropic dry etching such as reactive ion etching (RIE) and electron cyclotron resonance (ECR) plasma etching is widely used so as to reliably transfer a mask pattern on a layer such as a wiring layer to be processed. Such anisotropic dry etching uses plasma or ions.
A plasma process is likely to produce an electrical stress such as a damage caused by non-uniformity of plasma (refer to J. Appl. Phys. 72 (1992) pp.4865-4872). As patterns are becoming finer, the gate insulating film of an insulated gate type field effect transistor (IGFET) is becoming thinner. There are many gate insulating films which have a thickness of 10 nm or less and are susceptible to influences and damages by an electrical stress. For example, if a Fowler-Nordheim (FN) tunnel current flows through a gate insulating film, defects corresponding in amount to an integrated current flow are formed so that the threshold voltage changes. If a dielectric breakdown occurs, the gate electrode and semiconductor substrate are short-circuited.
A gate oxide film having a thickness of 10 nm is full of danger of breakdown when a voltage of 10 to 15 V or higher is applied. In a plasma atmosphere, a potential Vdc at the surface of a layer to be processed reaches 100 to 1000 V. It is not easy to set a uniformity of potential distribution 5% or less.
Therefore, there is a high danger of breaking a gate insulating film during a plasma process. This danger is present not only when patterning a wiring layer, but also when opening a contact hole or when cleaning a contact hole by plasma sputtering.
Conventionally, such a damage phenomenon has been considered to be resulted from non-uniformity of the electric or magnetic property of plasma. Therefore, as a means of eliminating damages, it has been endeavored to generate and use uniform plasma.
More specifically, it has been proposed to uniformize a bias voltage by providing a uniform plasma potential and a position independence of an electron mobility. For example, in a configuration that magnetic fluxes traverse over the surface of a layer to be processed, it has been proposed not to change the vertical components of a magnetic field between the surfaces of a central area and a peripheral area of a layer to be processed.
The present inventors have found that damages may be generated depending upon a type of patterns to be processed, even if the non-uniformity of plasma is improved.